As mentioned above, would it be safe to drive with a 14.6 foot long (90 lbs) canoe on top of my Toyota Corolla? The Canoe is just about as long as the car. I have a roof rack with crossbars, as well as all of the necessary tie down straps, but my concern is potential lift being caused while I am driving...
Is it safe to drive on the highway with this sort of setup?
If it is the length of your car, and you have tie down straps you should be able to do this perfectly safely. Ideally, get yourself canoe brackets to attach to your roof bars - these are curved to match your canoe and allow you to put more tension in the straps without damaging the canoe.
It is recommended (especially when you have one longer than your vehicle) that you also attach a strap at bow and stern if possible, as well as a warning flag or banner at each end. These straps will avoid excessive movement up and down, and the flag will help make others aware of the potential risk at head height (eg cyclists)
Some countries mandate flags at specific length of cargo overhang, others don't, so you should check your local regulations.
In addition to @RoryAlsop's answer I would add that you need to both tie it down to the roof rack and to the car at front and rear. The attachment to the car will help prevent wobble of the bow and stern of the canoe and help prevent it blowing the roof-rack off the car in (very) strong cross-winds - I have friends this has happened to, as unlikely as it seems.
In addition (as Rory alluded to) the bow/stern tie-downs help prevent wobble of the bow and stern while going over bumps in the road and the resultant stresses, which (I have heard - no proof) can result in micro-fractures in canoes made of fiberglass or similar materials - these materials are not common these days though.
